Abstract:
Invasive plants have the potential to reduce the diversity of species in plant and animal
communities. I examined the negative effect of two invasive wetland plants, purple loosestrife and reed
canary grass, on the species richness and diversity of plant and moth communities within 24 wetland study
sites in the Pacific Northwest. I hypothesized that as the cover of the invasive species increased, the
diversity of the local plant and moth community would decrease. Increasing cover of purple loosestrife and
reed canary grass was associated with reduction in the diversity of wetland plant communities irrespective
of the diversity measure examined. Moth species richness was positively correlated with plant species
richness, but I found no detectable direct negative association between loosestrife and canary grass cover
and moth community diversity. Wetland hydrology, soil characteristics, and topography were measured to
control for potentially covarying and confounding influences on plant diversity. Temperature, ambient
light, and surrounding land-use were measured to control for potentially covarying and confounding
influences on moth sampling and diversity. None of these variables was significantly associated with
invasive species abundance. This strengthens the conclusion that the invasive species are the cause of the
decline in biotic diversity.
Understanding the mechanisms that influence plant invasions will lead to more effective
management strategies. I examined the role of soil nutrients in the invasive potential of purple loosestrife.
I hypothesized that nitrogen was the primary nutrient limiting plant growth and that higher soil nitrogen
concentrations would increase the growth of purple loosestrife within 13 wetland sites in the Willamette
Valley, Oregon. Using greenhouse experiments and field studies I found that nitrogen was the primary
resource limiting both plant community biomass and purple loosestrife growth. Purple loosestrife grew
well in soils taken from nine wetlands currently not colonized by loosestrife. Given their similar
hydroperiods, this suggests that these wetlands will be susceptible to invasion should loosestrife colonize.
Plant species richness was negatively associated with soil nitrate and ammonium concentrations, This
trend included invaded and non-invaded sites. Therefore, to prevent repeated invasions, management
strategies should consider methods for reducing soil nutrient concentrations, particularly nitrogen.
